This invention relates to the method and apparatus for reconstructing objects and/or tomographic images from their projections. This invention has particular application in the field of medical computerized tomography.
There has been a constant trend towards faster and more accurate computerized tomographic scanners. The earliest medical scanners consisted of a source of radiation, such as X-rays, and a detector, the two of which traversed across the body to be examined in a linear manner, then were rotated a few degrees and the traverse repeated. In order to take sufficient readings to reconstruct a tomographic image, several minutes were required. For medical tomography, this length of time was undesirable because it meant holding the patient, and in particular the organ to be examined, totally still for this period of time. This allowed scans to be done on relatively stationary organs, such as the brain, but was not amenable to producing cross-sectional images of rapidly moving organs, such as the heart. Such a system is illustrated in U.S. Pat. No. 3,778,614.
In the quest of greater speed, it was found that a fan-shaped beam of radiation could be used which would irradiate a plurality of detectors simultaneously. See for example U.S. Pat. No. 3,881,110. With this setup, the traverse motion could be eliminated and the sole motion could be the rotation of the source and detectors. This system increased the speed but also increased the number of detectors--a very expensive component. Further different detectors took reading through different parts of the body. To sum these together, it was essential that each detector be equally sensitive and remain equally sensitive or else appear that more or less radiation was being absorbed by that part of the body.
The next step in increasing the speed was to have all the detectors stationary in a single rotating source (see U.S. patent application Ser. No. 726,556, filed Apr. 24, 1985, assigned to the U.S. Department of Health, Education and Welfare). Although this increased the speed, it required detectors to be placed 360.degree. around the patient. Other variations of the stationary detector them have also been tried (see, for example, U.S. Pat. No. 4,031,395, the embodiment of FIG. 3 which has 360.degree. stationary detectors, 360.degree. of stationary X-ray tubes, and rotating collimator means). The geometry of this system is such that it is really a traverse and rotate system; but because detectors and X-ray tubes are stationary, it is able to function much more rapidly than the early moving detector and moving source traverse and rotate systems. But again, this system requires 360.degree. of detectors. Indeed, as is pointed out on page 7 of the article Reconstruction from Divergent Ray Data, by A. V. Lakshaminarayanan in "Technical Report No. 92", State University of New York at Buffalo, Department of Computer Science, January, 1975, it was believed that 360.degree. of views were required in any reconstruction system, such as those above, hence enough detectors to encircle the full 360.degree. of the patient circle.
The present invention is a major breakthrough because it recognizes for the first time that 360.degree. of views need not be taken in divergent fan beam geometry. Instead, the present invention recognizes that every point within the area to be examined need only be viewed from 180.degree. of angles in order to produce a complete set of projection data. This, in turn, enables the system to operate much faster since the X-ray source need only scan a little over 180.degree., and it eliminates nearly half the detectors that would be needed in a 360.degree. scan system.
The speed with which the patient was scanned was not the only concern with early computer tomographic devices for medical use. The earliest units were very slow in producing images and the images that they produced were not as sharp and clear as would be desired. The early traverse and rotate systems, in effect, took a series of density readings as they traversed and then filled sequential columns of a matrix with the sequential density readings. When the system rotated and traversed again, it would fill a second matrix. These matrices were then stacked, each rotated at their angle relative to the other, and the intensities at the corresponding point of each matrix, i.e. each vertical column of intensities, were summed. This was a slow system and less than accurate.
Then it was discovered that if such sum of intensities were at some degree modified by its surrounding intensities the image could be refined. However, these methods were even more time consuming, often requiring as much as fifteen minutes to transform the data into a tomographic image.
The next step towards speeding up the processing of data into images was to modify the intensity at each detector by the intensities read on the surrounding detectors before summing intensity values into the matrix. See, for example, U.S. Pat. No. 3,924,129. The system shown uses a number of geometrically derived approximations to make these modifications. Subsequently, a large number of articles have appeared which have used convolution theory to modify the intensity at each point as a function of its surrounding points. These all multiply the intensity data at a given point by a convolution function whose values are determined by the intensities read at surrounding points. The convolved intensity data is then stored in matrices for processing into an image representation. The convolution method is faster than the geometric method because functions are brought together as a unit rather than a series of individual point calculations. However, as pointed out above, it has heretofore been believed that the convolution interval need be from 0 to 2.pi. radians, i.e. that absorption intensities must be read through all 360.degree. around the circle surrounding the body. Again, the present invention will cut down the image processing time because it is only necessary to work with views which surround each point of the object to be viewed by 180.degree..
A further advantage of the present invention is that it includes a faster back projection system. The back projector works with data in the same order as the convolver, hence the back projector can back project the data into the image memory as it emerges from the convolver.
Another advantage of the present system is that each detector receives absorption data along paths passing through the entire body. Hence, if all detectors do not have precisely the same sensitivity, the differences will be averaged out without causing an error in the final tomographic image.
Another advantage of the present system is that it has a convolver function which provides a greater resolution, faster processing and simpler calculations all from fewer views.